1. Field of the Invention
This invention relates to an imaging device using a solid-state image sensor which can be utilized in the field of medicine or industry.
2. Description of the Related art
Recent advances in semiconductor technology have offered compact design and high performance of a solid-state image sensor such as a CCD. The solid-state image sensor has widespread use in the region of medical apparatus, for example, in such a manner that a compact TV camera housing the CCD is attached to the eyepiece section of an endoscope to diagnose the inward parts of the body through a TV monitor in the TV camera housing the CCD, an optical low-pass filter is in general use for elimination of moire. Such optical low-pass filters, for which birefringent plates are often used, are available as the combination of quartz filters serving as the birefringent plates with other filters, such as polarizing plates, and quartz filters only.
The former optical low-pass filter comprising the combination of quartz filters and other filters is disclosed by Japanese Patent Preliminary Publication Nos. Sho 58-100802 and Sho 58-70686 and Japanese Patent Publication No. Sho 51-7381. These prior art articles employ .lambda./4 plates, depolarizing plates, optically active plates, and thin quartz filters, together with quartz filters, in order to equalize the separation intensities of spaced spots where the crystal axes of the quartz filters are oriented at angles other than 45.degree.. In each of such cases, however, numerous members constituting the low-pass filter have caused defects that the structure of the filter is complicated and the whole becomes bulky.
The latter optical low-pass filter comprising quartz filters only, on the other hand, is usually designed so that the crystal axes of plural quartz filters are oriented to shift, 45.degree. for each, and the separation intensities of spaced spots of the quartz filters are nearly equal. In this connection, however, various proposals have been made. For example, as disclosed by Japanese Patent Preliminary Publication No. 57-39683, the orientations of crystal axes of quartz filters are shifted 90.degree. and thereby four spots are formed through separation in a horizontal direction by three quartz filters. Further, as in Japanese Patent Preliminary Publication No. Sho 60-164719, three quartz filters, although oriented at 45.degree., are used to consciously superpose spaced spots for required low-pass characteristics. In Japanese Patent Preliminary Publication No. Hei 2-250572, four or more quartz filters oriented at 45.degree. are used to superpose spaced spots. These techniques, however, are such that the spaced spots are merely superposed geometrically for combination. Other methods of making the separation intensities of spaced spots nonuniform, as set forth in Japanese Patent Preliminary Publication No. Sho 61-501798 and Japanese Patent Publication No. Sho 58-8483 (which corresponds to U.S. Pat. No. 4,575,193), are that in order to make the separation between spots vary with colors, namely wavelengths, the low-pass characteristic is changed in accordance with each color.
When the number of birefringent plates constituting the filter is represented by N, the number of spaced spots m is expressed by EQU m=2.sup.N ( 1)
Specifically, as the number of birefringent plates is increased, the number of spaced spots increases, and the frequency of occurrence of the superposition and approach of the spaced spots becomes high. In this case, the separation intensities of spaced spots may differ with the relationship between the superposition and the phase difference of light waves at the spaced spots, and also with wavelengths (colors). Consequently, the required filter characteristics are not obtained, with the failure of moire elimination.
The above prior art, however, does not in any way discuss the relationship between the superposition and the phase difference of light waves and the wavelength dependence where a plurality of quartz filters are constructed so that the orientations of their crystal axes are shifted, 45.degree. for each. Thus, even though the optical low-pass filter is fabricated based on the design value, the fact will remain unnoticed that the required filter characteristics are not obtained due to the superposition and approach of the spaced spots caused by the increase of the number of spaced spots. As such, whenever trouble is produced, the filter has been discussed and redesigned.
In particular, where the TV camera using the solid-state image sensor is mounted to the fiberscope of the endoscope, moire is liable to occur because of an orderly array of fibers, and many quartz filters are required compared with common TV cameras used in the fields other than the endoscope. In this case, the larger the number of quartz filters, the higher the frequency of occurrence of the superposition and interference of the spaced spots. It has thus been difficult to derive the required filter characteristics. Consequently, moire elimination is so incomplete that the endoscope picture develops moire, and there is the fear of overlooking affected parts to cause an erroneous diagnosis.